Many agents will attempt to sell serial rights to the books that they work on, where appropriate: so, they'll try to place extracts in newspapers or magazines. But only to publications where there's a decent amount of money available because it really isn't worth their while placing articles which are only going to earn £50 or so. The nicest deal I've ever had in this regard was a three-day serialisation in the Daily Mail, which brought in £10,000 (less agency commission).
Similarly, with web content: it's notoriously poorly paid, and few agents can afford to work in this area. There's just no motivation for anyone to do it, unless the writer wants to press ahead with it alone.
Games publishers have their own dedicated writers to develop new titles, and only license tie-ins to the biggest-selling products; or they're invited in to work on a game as a film spin-off, for example. So they're very unlikely to pay a decent amount for the rights to a book.
Book publishers' marketing departments will sometimes place articles in national or specialist press on their authors' behalf, but then those articles have to be written and are often done for little or no pay.
Is that a help? If I've missed anything out do please ask again.
